The present invention relates generally to vehicle powertrains, and more particularly, to a front-wheel drive transmission which integrates a continuously variable transmission and a four-wheel drive transfer case into a common assembly.
In general, power transfer mechanisms, such as transfer cases, are operatively coupled to both manual and automatic transmissions for selectively directing the power to the non-driven wheels of a motor vehicle for establishing four-wheel drive modes. Many automotive transfer cases are equipped with a speed reduction gear assembly for providing xe2x80x9chighxe2x80x9d and xe2x80x9clowxe2x80x9d speed ranges in conjunction with two and four-wheel drive modes. As such, a range shift mechanism is provided for operatively shifting components of the speed reduction gear assembly between a high range position providing a direct ratio drive connection and a low range position providing a reduction ratio drive connection.
Conventionally, transfer cases are provided in combination with standard automatic and manual transmissions. However, conventional automatic and manual transmissions are heavy and contribute greatly to the weight of the vehicle and are only capable of establishing discrete gear ratios. Furthermore, automatic transmissions are inefficient due to the requirement that the hydraulic system utilized for controlling the shifting of the automatic transmissions require a large amount of power in order to maintain the hydraulic fluid pressure even when shifting is not necessary.
Accordingly, it is desirable in the art of automotive transmissions to provide a combined transmission and transfer case which has improved efficiency, continuously variable operating ranges, and reduced size and weight.
Accordingly, it is an object of the present invention to provide a four-wheel drive transmission which combines a transmission section and a transfer case unit into a common assembly.
It is another object of the present invention to provide a four-wheel drive transmission which is lighter in weight and more compact than conventional transmission and transfer case combinations.
It is still another object of the present invention to provide a front-wheel drive transmission which does not require an engine clutch.
These and other objects of the present invention are obtained by providing; a four-wheel drive transmission having a continuously variable transmission unit a, a continuously variable range unit, and a torque transfer unit integrated into a common assembly. Both the transmission unit and the range unit are equipped with a worm/worm gear transmission which are controlled by a transmission controller to provide continuous control of the transmission unit and the range unit.
Enveloping worm/worm gear transmissions with a worm gear having less than twenty-four teeth have not been commercially used because it was believed impossible to build such a transmission due to the need to undercut the root of the worm gear tooth. Thus, those skilled in the art did not consider enveloping type worm gears with less than twenty-four teeth to be feasible for commercial applications. In contrast, the enveloping worm/worm gear transmissions of the present invention utilize a worm gear without undercut gear teeth because of a greater enveloping angle for one revolution of the worm thread. With less than twenty-four gear teeth and a greater enveloping angle for one revolution of the thread, as compared to prior enveloping worm/worm gear transmissions, the minimum ratio for one thread can be reduced to two, with an increased in efficiency. In contrast, prior enveloping worm/worm gear transmissions had a minimum ratio of twenty-four for one thread of the worm. The efficiency of the new worm/worm gear transmission is even greater than that of well-known hypoid gearsets which are used in low ratio right angle drives. Thus, the present invention can replace hypoid or bevel gearing in many applications by reason of the low ratio. In addition, this new worm/worm gear transmission is able to back drive by transmitting torque from the worm gear to the worm. For the same size, this invention has more than twice the capacity of traditional hypoid gearing.
Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood however that the detailed description and specific examples, while indicating preferred embodiments of the invention, are intended for purposes of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.